`nbgv get-version -p path/to/project.csproj` fails on managed git repo

[AArnott]

When pointing nbgv get-version directly at a project file (as opposed to a project directory) it fails with:

Nerdbank.GitVersioning.GitException: Got a blob instead of a tree when opening object b9e7890ccc1892e6bb74993e1e6e4c27c3fea062

But this worked with libgit2. The problem seems to be that the managed implementation drills all the way into the project file and expects it to be a directory path. I don't know what the libgit2 implementation did differently to avoid failing, but for usability we should make this either work or produce a more useful error message.